Angle grinder gearboxes are convenient to fix wheels on to but they don't have enough gear reduction to be useful. If you use a cordless angle grinder motor or any typical small motor that other builders use, the wheels will be going far too fast. That's why so many people use cordless drill motors as they have the right amount of gear reduction built in.

When the guys are asking what grade of stainless, they mean which alloy. There are only two common stainless alloys available in thin sheets - 304 and 316. For armour, there really isn't much between them - they are both relatively soft and low strength compared to other steels. Here is a comparison chart of stainless steel properties: http://www.mcmaster.com/#about-stainless-steel/=ycr5gp. The steel that would make good armour is towards the bottom of the table and is insanely expensive.